World wide weather

Dec 17, 2010

Ken McKinley

Many voyagers are quite familiar with the marine weather products available from U.S. sources. These include text weather summaries, analysis charts showing the current weather pattern, text forecasts, and forecast charts showing the expected weather pattern at a future time.

What happens, though, when as a voyager you move beyond these areas — say into the Mediterranean, the North Sea, or the Baltic Sea; perhaps you are heading through the South Pacific toward New Zealand or Australia, maybe even sailing around the southern portions of Africa or South America. In all of these cases, it will be necessary to use weather resources from other countries, and this means that in addition to learning about different cultures and customs in these areas, and the different weather patterns that may prevail, you will also need to use weather products that don’t look quite the same as the ones you are used to from U.S. sources. The good news is that there are many reliable products available from other nations — the bad news is that sometimes they are a bit difficult to locate, and the formats will be different than the familiar U.S. versions.

There are some aspects of the products that are similar from nation to nation, and we will start by focusing on those. First, when dealing with forecasts of wind speed, the values that are given will always be those for sustained winds, that is average wind speeds that persist for at least several minutes, and often longer. Sustained wind speeds do not include wind gusts, which are very short-term (usually less than a minute) increases in wind speed. In text forecasts, sustained wind speeds are usually given as a range (e.g. 15 to 20 knots) and sometimes a separate value is provided for gusts. If a gust value is not given, then mariners need to assume that higher gusts than the sustained values will be experienced at times. For chart products where wind speed and direction is shown graphically using wind barbs, gusts are not indicated, just sustained winds, and again, mariners must infer higher gusts.

Second, when information is given about wave heights, either in text forecasts, or in graphic form on charts, the values given are for significant wave height, which is defined as the average of the highest one third of all the waves present. This means that higher (and lower) waves will be experienced at times, and generally the extreme wave height (approximately 1 in every 100 waves) will be twice the significant wave height.

Units of difference
With these definitions in mind, we need to look at the differences that will arise between countries, and the first of these is the units that are used to measure winds and seas. Different countries use different units, and it is essential that mariners are aware of which units are being used for the products they are consulting. As an example, five-foot seas are a lot different than five-meter seas!
Another difference that may arise is the units in which barometric pressure are presented, particularly on charts. The two major units here are millibars (mb) and hectopascals (hPa). Happily, even though these two units have very different names, their numeric values are identical. That means that 996 mb is the same as 996 hPa. Occasionally in text products, other units of barometric pressure may show up, such is inches of Mercury (inHg), or millimeters of Mercury (mmHg), but these are rarely used on charts.

Speaking of surface pressure charts, one of the main uses of these charts by mariners is to determine wind speed and direction (or forecast wind speed and direction) for a given location. This can be done by examining the pattern of the isobars and using Buys Ballot’s law, which states that if one stands with one’s back to the wind, lower pressure will be located to one’s left in the northern hemisphere (or to the right in the southern hemisphere). To employ this rule with a surface pressure chart, choose a point anywhere on the chart, and imagine that you are standing at that point with your left hand extended out from your body and pointing toward the adjacent isobar with the lower pressure value. The wind direction will then be coming approximately from your back. Figuring the wind direction this way will yield a direction parallel to the isobars, but due to surface friction, the wind will typically not be exactly parallel, but rather will be across the isobars at an angle of anywhere from about 25° to 45°, so a slight modification of Buys Ballot’s law is needed.

The determination of wind speed can be done using the spacing between adjacent isobars. The closer the isobars are to one another, the stronger the wind speed will be, and, conversely, the farther apart they are, the lighter the winds will be. Some surface charts have a scale on them to convert this distance between isobars directly to a wind speed, but on most charts (including all U.S. charts) there is no scale, and mariners must either use a more complicated external scale, or make an estimate based on actual wind information shown elsewhere on the chart, at about the same latitude as the point of interest.

An important aspect of surface pressure charts is the contour interval between isobars. For most U.S.-based charts, this contour interval is 4 mb. This means that isobars are drawn every 4 mb (e.g. 996 mb, 1,000 mb, 1,004 mb, etc). Occasionally, especially in tropical areas, intervening 2-mb isobars are included on U.S. charts, but when this occurs, they are drawn as dashed lines rather than solid lines so it is easy to tell them apart.

In other parts of the world, though, charts are produced with contour intervals of 2, 3, 4, or even 5 mb. The use of these charts is basically the same as the U.S. charts, but the mariner needs to be aware of what the contour interval is so that a proper determination of wind speed can be made. For example, if one was comparing surface pressure charts from nearby countries, and one had a contour interval of 4 mb, and the other 2 mb, the chart with the 2-mb contour interval will have twice as many isobars, meaning that the isobar spacing will be only half as much. At first glance, such a chart might appear to be indicating much stronger wind speeds due to the closer spacing of the isobars, but when the contour interval is considered, the two charts will indicate the same wind speed.

Charts available outside U.S.
Now that we have gone through some of the basics of how to use text forecast and charts, let’s see what can be found around the world. Even though many products from foreign countries are available through weatherfax and/or voice broadcasts, I am going to focus on what can be found on the Internet, since most high-seas voyagers now have Internet connectivity on board. One of the best tools to find weather information through the Internet is a search engine (Google, Yahoo, etc.), and the best parameters to search with are “country meteorological service” where one inserts the name of a country adjacent to the waters of interest in place of the italicized word. This will usually produce the official website of the national meteorological service of the country (if one exists) within the top few results. If this does not produce the desired results, try a neighboring country.

Once you have reached the website of a national meteorological service of your area of interest, you will need to determine if the website produces information in a language you can understand. Some websites offer their content in both the native language of the country and in English, so look for a link to an English version. If there is not a link, it may still be possible to get some understandable information, especially charts which rely less on text information. Look for the valid time(s) of any forecasts or charts to make sure you are accessing the appropriate information. It is important to keep in mind that these times may be given in either UTC (Universal Time, also known as GMT, or Zulu time) or in the local time of the country. It is a good idea to be familiar with the time zone of the area of interest.

It is also important to realize that the national meteorological services of most countries will concentrate on land-based weather information, and this is what will be featured most prominently. For maritime nations, there will usually be a link to weather information tailored toward those at sea, but sometimes it is not obvious. Also, not all countries provide all of the information at no cost. Some countries provide basic information for free, but also offer a more comprehensive service available by paying a fee of some sort. This value-added information may be similar to that available from the private meteorological industry in the U.S., and may include additional forecasts and/or charts, and sometimes custom forecast services are available. An accompanying table provides web addresses for several national services around the world that provide marine weather information, but the list is not exhaustive, so use your search engine to look for exactly what might be needed.

Many foreign governments utilize some of the same computer model data used in the U.S., and some websites will provide this data in graphic form. This is basically GRIB data, which has become widely available in many different forms on the Internet and through software packages. With all of this data, it is very important to realize what it offers, and what it does not offer. This type of data is pure computer model output from one model, and has not been reviewed or altered by a meteorologist. It will not take into account other computer models, and more importantly, will not factor in any local effects which may prevail in certain parts of the world. In general, mariners are better off using products which have been prepared by a professional meteorologist and therefore incorporate more comprehensive data, as well as the knowledge and experience of the meteorologist.

If you are planning a voyage far from U.S. waters, do some research ahead of time to locate the web-based products that will meet your needs. It is much easier to do this searching with a fast connection and in the comfort of your home so that when you are underway on board, perhaps in a less than perfectly-comfortable situation and with a slower connection, you will be able to use bookmarks to your pre-determined webpages to get the needed products more easily and quickly. Also, by researching available products online ahead of time, you will be familiar with products which may be available through radiofax or other means.

Ken McKinley is a trained meteorologist and the owner of Locus Weather in Camden, Maine.